The global positioning system (GPS) uses satellites in space to locate objects on earth. GPS uses L-band signals from the satellites, which are tracked by a GPS receiver and used to determine the position of the GPS receiver. Currently, two types of GPS measurements are available within a civilian GPS receiver for each carrier signal of each GPS satellite that is being tracked. The two types of GPS measurements are pseudorange, and integrated carrier phase. These two types of measurements are available on each of two carrier signals, L1 and L2, with frequencies of 1.5754 GHz and 1.2276 GHz, respectively. The wavelengths of these two frequencies are 0.1903 m and 0.2442 m, respectively. The pseudorange measurement (or code measurement) is a basic GPS observable that all types of GPS receivers can make. It utilizes the C/A or P pseudorandom codes modulated onto the carrier signals. The pseudorange measurement records the apparent time taken for the relevant code to travel from the satellite to the receiver, i.e., the time the signal arrives at the receiver according to the receiver clock minus the time the signal left the satellite according to the satellite clock. The carrier phase measurement is obtained by integrating a reconstructed carrier of the signal as it arrives at the receiver. Thus, the carrier phase measurement is also a measure of a transit time difference as determined by the time the signal left the satellite according to the satellite clock and the time it arrives at the receiver according to the receiver clock. However, because the initial number of whole cycles in transit between the satellite and the receiver when the receiver starts tracking the carrier phase of the signal is not known, the transit time difference obtained from the carrier phase will typically be in error by multiple carrier cycles, i.e., there is a whole-cycle ambiguity in the carrier phase measurement.
The range or distance between a GPS receiver and each of a multitude of satellites is calculated by multiplying each signal's travel time by the speed of light. These ranges are usually referred to as pseudoranges (false ranges) because the receiver clock generally has a significant time error which causes a common bias in the measured range. This common bias from receiver clock error is solved for along with the position coordinates of the receiver as part of the normal navigation computation. Various other factors can also lead to errors or noise in the calculated range, including ephemeris error, satellite clock timing error, atmospheric effects, receiver noise and multipath error. In standalone GPS navigation, where the receiver obtains code and/or carrier-phase ranges from multiple satellites without the benefit of corrections from any reference stations, the receiver is very limited in methods available to reduce the errors or noises in the ranges.
To eliminate or reduce systematic errors, differential operations are typically used in GPS applications. Differential GPS (DGPS) operations typically involve one or more reference receivers located at known sites (sometimes called base stations) together with a communication link between the user receiver and the reference receivers. The reference receivers are used to generate corrections associated with some or all of the above error types and these corrections are sent to the user receiver over the communication link. The user receiver then applies the corrections to its own measurements or position and thereby obtains a more accurate computed position. The corrections from a respective reference receiver can be in the form of corrections to the reference receiver position determined at the reference site or in the form of corrections to the specific GPS satellite clock and/or orbit. Differential operations using carrier-phase measurements are often referred to as real-time kinematic (RTK) positioning/navigation operations.
The fundamental concept of Differential GPS (DGPS) is to take advantage of the spatial and temporal correlations of the errors inherent in the GPS measurements. Thus, the corrections cancel or significantly mitigate most of the noise sources in the pseudorange and/or carrier phase measurements. The amount of mitigation depends upon the correlation between the error sources at the user and reference receiver. While the GPS satellite clock timing error, which appears as a bias on the pseudorange or carrier phase measurement, is perfectly correlated between the reference receiver and the user receiver, most of the other error factors are either not correlated or the correlation diminishes as a function of distance between them.
To overcome the error sources within the DGPS system in wide-area applications, various regional, wide-area, or global DGPS (sometimes referred to as WADGPS) techniques have been developed. The typical WADGPS includes a network of multiple reference stations in communication with a computational center or hub. Error corrections are computed at the hub based upon the known locations of the reference stations and the measurements taken by them. The computed error corrections are then transmitted to users via a communication link such as satellite, phone, or radio. By using multiple reference stations, WADGPS provides more accurate estimates of the error corrections.
A number of different techniques have been developed to obtain high-accuracy differential navigation using the GPS carrier-phase measurements. The technique with the highest accuracy is the RTK technique, which yields a typical accuracy of about one-centimeter. In order to obtain that accuracy, however, the whole-cycle ambiguity in the differential carrier-phase measurements must be determined. When the distance between the user receiver and the reference receiver (baseline distance) is short, the RTK technique is highly advantageous because in this case, the whole-cycle ambiguity can be resolved not only accurately but also quickly. On the other hand, when the baseline distance is more than a few tens of kilometers, it may become impossible to determine the whole-cycle ambiguity and the normal RTK accuracy cannot be achieved. Another limitation of the RTK technique is that it requires a local radio link to be maintained between the reference receiver and the navigation receiver to supply timely correction or measurement data.
The WADGPS techniques that employ a carrier-phase differential method can also achieve very high navigational accuracy. The WADGPS differential techniques are also typically characterized by reliable long distance low-frequency communication links or by reliable satellite communication links. Thus, corrections can generally be communicated to navigation receivers without significant interruption. However, the WADGPS techniques usually treat the whole-cycle ambiguities as a real-valued (non-integer) variable and solve for a “floating ambiguity,” which is usually very poorly defined until measurement data covering a time interval of significant satellite geometry change have been obtained. Thus, in a WADGPS application, a time interval as long as one or two hours may be required to solve for the “floating ambiguity” with sufficient accuracy to yield a navigated position with an accuracy of less than (i.e., better than) 10 centimeters.